In the TDMA communications system, multiple earth stations share an identical carrier wave on a time division basis. Each earth station is allowed to transmit bursts (viz., high speed transmission of data bits) in a manner that each burst is located within an allocated time slot of each consecutive TDMA frame. The times of the bursts are carefully controlled so that no two bursts overlap.
Each burst transmitted from the earth stations usually contains, in its head portion or so-called "preamble", a fix pattern with a powerful self-correlating function. In order to detect the bursts, an arrangement provided for detecting bursts correlates a series of received signals with the fix pattern previously stored in the arrangement. Each time a definite correlation is obtained, the arrangment outputs a burst detection pulse.
A false burst detection, however, occurs due to thermal noises (for example). In order to overcome such a difficulty, it is a common practice to open, within each frame, multiple windows (or prediction gates) each of which is positioned such that the burst is received within the time duration defined by the window. Thus, the bursts detected within the window are deemed to be actual bursts and not noise.
As known, more than two kinds of windows are used in the TDMA system. For example, when a given earth station initially aquires bursts, a wide window is first used and thereafter a narrower one is employed. The narrow window is controlled in its position within a frame according to the burst position detected in the preceding frame. After the burst aquisition control is completed, another narrow window termed "burst sync window" is produced whose position is fixed within each frame.
FIG. 1 is a diagram showing a renewal time period of a window, wherein it is assumed that (a) only two earth stations A and B are involved in the communications system and (b) the station A receives one burst, within one frame, transmitted from the other station B. The station A successively monitors the receiving conditions of the bursts during a preset time period, and then determines a window mode each period T (during which the window mode thus determined is maintained).
FIG. 2 shows, in block diagram form, a known arrangement for detecting bursts in the TDMA system, in which it is assumed that a given station receives bursts from another on a one-burst within one-frame basis in a manner similar to the assumption made in connection with FIG. 1.
In FIG. 2, the signal which specifies burst positions within each frame, has already been applied, as burst time plan signal S3, from a source external of a burst position memory 80. A window generater 90 receives a window position signal S5 and generates a window signal S7 at a position determined by the signal S5. The window signal S7 has a mode which is determined by a burst condition signal Sl3 outputted from a burst condition decision circuit 100, and is applied to the next stage, viz., a window superimposer 20 which receives a correlation detecting signal S2 from a correlation detector 10. This detector 10 correlates the fix pattern stored therein with a received signal S1, and generates the signal S2 when a definite correlation is obtained therebetween.
The window superimposer 20 superimposes the window signal S7 and the correlation detecting signal S2, and permits the transmission of the signal S2 which occurs during the issuance of window signal S7 to pass therethrough. The output of the window superimposer 20 takes the form of a burst detecting signal S9. This signal S9 is then applied to the burst condition decision circuit 100, which counts the signals S9 to determine the burst condition (BR (Burst-Received) or BNR (Burst-Not-Received)) according to predetermined algorithms. The burst condition corresponds to the window mode one to one. The burst condition decision circuit 100 applies the output thereof (viz., S13) to the window generator 90 each period T. On the other hand, the window generator 90 continues to generate the same windows until the burst condition signal S13 changes.
The prior art discussed hereinabove with reference to FIGS. 1 and 2, is an oversimplified example in that there exists only one burst within each frame. However, in practice a plurality of bursts (the number of which is assumed N) is contained in one frame, in the cas of which the arrangement becomes complex and causes an increase in the number of circuit blocks (as shown in FIG. 3).
FIG. 3 shows, in block diagram form, a known arrangement for burst detection in the case where one frame contains N bursts. It should be noted that like blocks are denoted by like numerals and that some blocks bear suffixes (1 through N)) in this Figure as well as the arrangement shown in FIG. 2.
The arrangement shown in FIG. 3 is essentially identical to that shown in FIG. 2 except that the former arrangement includes N blocks 99.sub.1 through 99.sub.N (each of which is identical to a block 99 shown in FIG. 2) and a switch 110. In this Figure, the window position signal S5 is applied to the switch 110 which in turn applies the receive signals to the window generators 90.sub.1 through 90.sub.N in the chronological order of the burst within a frame. On the other hand, the correlation detecting signal S2 is applied to the window superimposers 20.sub.1 through 20.sub.N. Each of the blocks 99.sub.1 through 99.sub.N operates in the same manner as described with the FIG. 2 arrangement and hence further discussion will be omitted for brevity.
As will be apparent from FIG. 3 the illustrated arrangement has encountered the drawback that it becomes bulky, complex and expensive with the number of the bursts to be detected per frame. In general, in the TDMA communications system, it is necessary to detect up to several tens of bursts in one frame and hence tends to render the circuitry involved very complex.